RE: Correct GWS Flight Pack for Slow Stick?
Of course, one step at a time. The 6-cell 400mAh pack will get the plane flying, and it's easy to get (because it comes with the flight pack). Optional packs from third-party vendors will make the plane perform better, but it's often difficult for a newbie to electrics to source these because they generally don't know what all the numbers mean.
I'll stick my neck out and recommend a battery vendor: Cheap Battery Packs. You can go on their website, (BIG hint: take the company name and put a ".com" after it) and order a custom battery pack at a very reasonable price.
A 7 or 8-cell pack of their CBP650 cells is nice and light, gives the plane unbelievable power, and won't cost you a mint. Just keep the prop size down, a 10x8 at the largest, 9x7 is preferred for 8-cell packs.